Unable to set document field value to null (postgress-serverless-readmodel)

[alexslavr]

Describe the bug
Unable to set the field value to null using the postgress-serverless-readmodel adapter

Error message:
resolve:resolve-readmodel-postgresql-serverless:update-to-set-expression Either key includes "$set", "$unset", "$inc" simultaneously
resolve:resolve-readmodel-postgresql-serverless:update-to-set-expression Either key has children update nodes

To Reproduce

await store.defineTable('MyTable', {
      indexes: { id: 'string' },
      fields: [ 'a', 'b' ]
    })

await store.update('MyTable', { id: '1' }, { $set: { a: 'someValue', b: null }  })